The selection of materials for engineering applications requires characterization of the fracture behavior of these materials, since the reliability of the resulting product or structure depends on the materials' ability to resist applied loads and stresses. The properties most important for such applications are fracture toughness and fatigue resistance.
Numerous laboratory tests have been developed to determine the fracture toughness of a material by applying mechanical stress. The results of such tests have been used to rank materials for material selection by fracture toughness. However, these tests are performed at an isothermal temperature.
It is generally assumed that the fracture behavior measured by these tests describes the fracture behavior in situations where the stress is applied by heating or cooling. Temperature induced stresses are very common in areas such as aerospace, automotive, civil, electronic and consumer applications and occur in all parts or structures in which at least two materials with differing coefficients of thermal expansion (CTE) are present. During heating or cooling, the materials expand at differing rates thus experiencing gradual or incremental changes in loading and stress. There is experimental evidence that the fracture toughness measured by mechanically induced, isothermal testing inadequately describes the fracture behavior of parts that experience temperature changes.
In cases in which many materials are being screened for an application, it is time consuming to identify the exact fracture toughness of each material. Moreover, other properties such as the relaxation modulus and the CTE of the considered materials must be evaluated as well.
There remains a need for a method to measure the fracture toughness and fatigue resistance for materials for transient temperature environments and to rank such materials.